1. Field of the Invention
The present invention relates to a liquid crystal material, more particularly, to a super twisted nematic mode liquid crystal material by which a largescale display of 640.times.400 dots or more can be formed at a high duty ratio without inducing image crosstalk. The present invention also relates to a super twisted nematic mode liquid crystal display device using the liquid crystal material of the present invention.
2. Description of the Related Art
As is well-known, a variety of liquid crystal display devices are widely used as a display means in different fields of technology, due to their many advantages such as a high information content, low drive voltage, extreme durability, and a flicker-free display. Based on these advantages, the liquid crystal display devices are particularly used in terminals of information processors, especially office automation or "OA" devices, for example, personal computers or word processors, and when used in such OA devices, the display devices are generally driven by a simple matrix drive system using X- and Y-stripe electrodes, i.e., so-called "direct driving method".
A typical example of such prior art liquid crystal display devices is a twisted nematic or "TN" mode liquid crystal display device. The TN mode liquid crystal display device is characterized by a twisting angle of 90.degree. and is particularly useful in a black and white display because of an absence of any spectrum dispersion. Nevertheless, this type of the display device does not provide a satisfactory display of images on a large scale, as it has a duty ratio of at most about 1/64. If the duty ratio is increased to more than 1/64, the contrast of the images is deteriorated with the decrease of the duty ratio, and at the same time, crosstalk among the displayed images occurs. It is considered that the crosstalk is caused by the simple matrix structure of the device; i.e., the crosstalk is induced in this simple matrix structure device due to interference between each picture element.
More particularly, FIG. 1 shows a characteristic curve (applied voltage vs. transmittance at 1/64 duty ratio) of the prior art TN mode liquid crystal material. As shown in the FIGURE, a transmittance (%) can be varied depending upon the applied voltage (V), and a satisfactory difference of the transmittance at two different voltages can be obtained V.sub.on for selected voltage and V.sub.off for non-selected voltage. However, the curve also shows that, if the duty ratio is decreased to 1/200 or less to provide a large scale display, the difference of the transmittance is reduced, and thus crosstalk occurs, as the V.sub.off is close to the V.sub.on , because of a gentle inclination of the curve, and accordingly, a poor threshold characteristic is shown.
Recently, it has been found that the above problems of higher duty and crosstalk can be solved if the liquid crystal material and the orientation layer are improved. Surprisingly, a super twisted nematic or "STN" mode liquid crystal material, characterized by a twisting angle of more than 90.degree. and less than 270.degree., enables a large size liquid crystal panel of 640.times.400 dots to be driven at a duty ratio of 1/200 to 1/400, without image crosstalk. This is because the STN mode liquid crystal material has a good and steep threshold characteristic. Note, the STN mode liquid crystal material generally comprises a mixed nematic liquid crystal and a cholesteric phase liquid crystal added to the mixed liquid crystal to give a helical structure thereto.
It has been also found that, although the STN mode liquid crystal material is generally considered to be satisfactory, notable crosstalk sometimes occurs therein depending upon specific images or patterns to be displayed, and this crosstalk is increased with an increase of the information content.
The inventors carried out an intensive study of the reasons why the crosstalk occurs in the prior art STN mode liquid crystal material, and found that the crosstalk is increased with an increase of the number of pulses needed for writing images or patterns in the liquid crystal material. Namely, a brightness in a background area of the display is not the same in areas of complicated patterns (increased number of writing pulses) and in areas of no substantial pattern (the least writing pulses).
From the above study, the inventors also found that the crosstalk occurs in the STN material because an effective voltage applied to the liquid crystal material can be changed depending upon the length of the writing pulses, i.e., frequency of the writing pulses, and the characteristic curve of the applied voltage vs. transmittance is accordingly shifted to the left (lower voltage) or right (higher voltage). The mechanism of the crosstalk is diagrammatically shown in FIG. 2, in which the frequency of the writing pulses is 50 Hz, 1 kHz and 10 kHz. When the effective voltage applied to the liquid crystal material was changed as a result of an increase of the frequency, the characteristic curve was shifted as shown in FIG. 2. The details of this mechanism will be discussed hereinafter with reference to FIG. 3 and 4, for a further understanding of the present invention.
Therefore, there is an urgent need to provide an improved STN mode liquid crystal material not having the prior art problems described above, such as crosstalk, and capable of displaying images or patterns in a large display area. Note, crosstalk must be completely prevented regardless of the pulses used when writing.